This invention relates generally to optical scanning systems and particularly to an optical scanning system capable of determining the angular deviation of each point defining a scanned object from the boresight axis of such system.
A scanning system of the type herein contemplated generally comprises an optical system for focusing a portion of the electromagnetic energy radiating from objects within a field of view into a corresponding field containing images of such objects on a focal plane; at least one detector element disposed in such corresponding field and responsive to such focused electromagnetic energy to produce an electric signal analogous to each image; a scanning mechanism for cyclically moving the at least one detector element relative to the corresponding field in a prescribed manner; and a processor responsive to electric signals thereby produced to derive signals indicative of the position of each one of the objects within the field of view. Depending on the application of the system, the desired information may take various forms, ranging from the measurement of the relative angular position of each scanned object with respect to the scanning system to a two dimensional visual reproduction of each scanned object. The former type of information is useful in a basic guidance system for guiding a missile towards a target, while the latter type of information is useful in more sophisticated guidance systems where, say, a measurement and identification of the plume of the jet exhaust of a target aircraft is required for improvement of the kill probability of the missile system.
The precision and speed of operation of a scanning system are, inter alia, functions of the number of detector elements scanning a field of view and the rate at which such field is scanned. Therefore, in applications where the required precision and/or speed of operation of the scanning system makes it necessary to use more than one detector element, a linear array of detector elements has frequently been employed, each one of such detector elements being responsive to electromagnetic energy from a different part of the field of view. Thus, a scan pattern using such a linear array may be formed by disposing the detector elements in a line parallel to one dimension of a two dimensional field of view and translating such detectors across the second dimension of the field of view. One scheme typically employed is to drive the detectors in periodic or sinusoidal motion back and forth to scan the field of view; however, since it is desirable to have a substantially constant scan rate the linear array must be moved beyond the limits of the prescribed field of view. Another scheme to achieve a substantially constant scan rate is to use a mechanical cam arrangement. A difficulty of such a cam arrangement is that high accelerations are required at the edges of the scanned field of view. In order to reduce such high accelerations, it is known to dispose a linear array of detector elements in a line parallel to one selected dimension of the field of view and rotate such linear array about an axis coaxial with the center of the field of view. In such a scan pattern the orientation of the linear array of detector elements is always maintained parallel to the selected dimension of the field of view. This latter scan pattern, however, has the disadvantage of scanning the imaged object in the direction parallel to the array during a portion of the scan cycle, thereby reducing object resolution and increasing processing complexity. With a "cross" array arrangement (wherein a second linear array of detector elements is disposed orthogonally to a first linear array of detector elements) object resolution is improved at the price of adding still more complexity to the processor.